Light water cooled and moderated reactors, and particularly PWRs, are generally provided with clusters of neutron absorbing rods, for controlling operation of the reactor. Each cluster is arranged for being securely connected to a drive shaft extending upwardly from the cluster. The drive shaft is associated with a drive mechanism suitable for linearly moving the shaft, whereby the cluster may be introduced into the core by an adjustable amount and removed from the core. The drive mechanism is typically placed above the cover of the pressure vessel of the reactor and is associated with a stationary sleeve projecting through the cover and accomodating the drive shaft, which is connected to the cluster by a disconnectable coupling.
In most PRWs, the vessel contains a stationary structure for supporting and holding the core and for guiding the clusters and their drive shafts. The latter function is fulfilled by an assembly belonging to the reactor structure, placed above the core and usually called "upper internals". The upper internals constitute a rigid structure comprising a guide tube for each cluster, each guide tube being in alignment with one of sleeves.
It is periodically necessary to replace some of the fuel assemblies or to change their location in the core. For that purpose, the control clusters (that designation including possible spectrum shift clusters) are lowered into the core by actuation of their drive shafts. The vessel cover is removed for providing access to the upper internals. Each drive shaft is separated from the corresponding cluster using an axially introduced remote control tool. Then the upper internals can be raised along with the drive shafts and provide access to the fuel assemblies.